When available in the appropriate amounts, selenium appears to be incorporated specifically into functional proteins of the cell. Higher concentrations, however, are toxic to most living systems. This toxicity depends on many factors such as its chemical form, animal or plant species, diet, the presence of certain additives, etc. These studies should give a better understanding of selenium biochemistry and toxicity at the molecular level in seleniferous plants (Astragalus and Stanleya) and nonaccumulator organisms. The work will provide a greater insight into the metabolic pathways that are used by the selenium- containing analogs of the sulfur amino acids (i.e., selemocysteine and selenomethionine) that lead to their incorporation into proteins. This includes an examination of the details of the aminoacylation reaction and the steps leading up to and including peptide bond formation. Several possible control mechanisms will be examined that selenium accumulator species may use to limit the extent to which selenium analogs of the sulfur amino acids are incorporated into cellular protein. Nucleic acids from seleniferous plants will be examined for the presence of naturally occurring selenium-containing purines and pyrimidines by ion-exchange and ion-exclusion high pressure liquid chromatography. The methylated nucleoside content of transfer and ribosomal RNAs will also be examined using similar methodologies, in addition to the Randerath tritium labeling procedure.